Microwave dryer for ceramic articles

ABSTRACT

A process and an apparatus are provided for drying molded ceramic objects using a combination of microwave heating and air ventilation. The objects are heated rapidly in a first microwave cavity oven while applying a relatively light flow of air. The objects are then maintained at a constant temperature in a second microwave oven while applying a heavy flow of air to evaporate water. The oven cavities are in a common enclosure divided by a partition with separate magnetron generators mounted in each portion. Ventilating ducts connected to an air source supply air to the cavities. Molds carrying the objects are fed by a belt into each cavity in a controlled heating and drying sequence to produce large quantities of ceramic objects with minimum cracking and warping. The power and space requirements are also reduced.

Guerga et al.'

MICROWAVE DRYER FOR CERAMIC ARTICLES [72] lnventorsr Michel Henri Guerga, Clichy sous Bois; Bernard Lucien Desire Hallier, La Varenne, both of France [7 3] Assignee: International Standard Electric Cor- I poratiou, New York, N.Y.

[22] Filed: Jan. 11, 1971 [21] Appl. No.: 105,391

301 Foreign Application Priority ooa Jan. 14, 1970 .France ..'...7001187 [52 us. Cl. ..34/ 1, 219/10.5S [51] Int. Cl. ..Blk 5/00 ['58] Field of Search ..34/1

[56] References Cited UNITED STATES PATENTS 7 3,409,447 11/1968 Jeppson ..'....34/l 3,434,220 3/1969 Forster ...34/1 3,460,265 8/1969 Smith ...34/l 3,474,544 10/1969 Holden et a1. ..34/l

3,704,523 Doc. 5, r972 Primary Examiner-Carroll B. Dority, Jr.

[57] ABSTRACT A process and an apparatus are provided for drying molded ceramic objects using a combination 'of microwave heating and air ventilation. The objects are heated rapidly in a first microwave cavity oven while applying a relatively light flow of air. The objects are then maintained at a constant temperature in a second microwave oven while applying a heavy flow of air to evaporate water. The oven cavities are in a common enclosure divided by a partition with separate magnetron generators mounted in each portion. Ventilating ducts connected to an air source supply air to the cavities. Molds carrying the objects are fed by a belt into each cavity in a controlled heating and drying sequence to produce large quantities'of ceramic objects with minimum cracking and warping. The power and space requirements are also reduced.

9 Claims, 10 Drawing Figures III [III/III IIII Illri 'QIIIIII. III/[IA P'A'TE'N'TEDuEc 5 m2 SHEEI 1 0f 4 lnvenlors MICHEL GUERGA OCRNARD L. D. HALL/ER By I A Home PATENTEDBEC 5 mn- SHEET 2 0F 4 l nncnlrs menu. a. cuancn nan/V400 L. a. HALL/ER & Horne v7 PATENTEDMB 91 3.704.523

sum u ur 4 3/ 32 fig) 35 33*{ J r I i 34 4 34 4 35 *7 55 52 53 I %E J Inventor: rue/v54 H. GuenqA BERNARD L. D. HALL/ER .4 Home MICROWAVE DRYER FOR CERAMIC ARTICLES BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention concerns a process and equipment for microwave heating principally for the predrying of ceramic pastes used for the manufacture of chinaware pieces of moderate dimensions such as plates, dishes or electric ceramic pieces. The source of microwave energy is constituted by tubes such as magnetrons or klystrons operating at certain frequencies reserved for industrial heating, for example 2450 Mc/s.

2. Description of the Prior Art In the series of production stages of chinaware there exists an important link, that of the predrying of the plastic paste. This is an operation which in the past needed a great deal of time. In modern manufacture,

with large numbers of output pieces, use of the prior processes would result in the predrying stage becoming a production bottleneck. In present porcelain techniques, after shaping the paste in a plaster mold carrying theimpression of the final object, using a special device, such as the Roller machine, which is well known to those in the ceramic art, it is necessary to ensure the drying of thepaste. During a first phase, water escapes at the same time as the object undergoes shrinkage. After the elimination of a certain quantity of water, the paste continues to dry but the loss of water is no longer accompanied by shrinkage. The first phase, called predrying, is a delicate stage of the manufacturing sinceit is necessary that during this time there be no accidents such as cracks or warping.

- In traditional techniques, the predrying is carried out in free air, the water in the paste being eliminated as much by slow evaporation as by absorption in the pores of the mold. The operation was completed when the object could be taken from the mold without effort and possessed sufficient rigidity to be placed on an appropriate form with a view to complete drying. The predrying was a lengthy portion of the manufacture which required much handling, extensive workshops with numerous stages, and quantities of molds the stock of which had to be renewed quite rapidly due to progressive fouling of the plaster and its decomposition under the effect of the absorbed moisture.

Porcelain manufacturers have sought to reduce the duration of the predrying by subjecting the molds to infrared radiation and/or a flow of dry hot air. The evaporation of water, however, is a surface phenomenon which forms a surface crust preventing the elimination of the water from the interior of the paste. Shrinkage is badly effected and cracks and warpings appear. In order to remedy this defect, certain manufacturers have attempted to place the objects and molds in a current of hot and humid air. The operation actually becomes longer and does not offer any advantages over the traditional method of predrying in free air.

An analysis of the process of predrying shows that if the humidity is evacuated unequally at the surface of the paste and in the internal layers, contractions and tensions would result which are unequally distributed in the paste and thus cause permanent cracks and warping. A uniform transfer of the moisture and an even shrinkage at the surface and in the interior of the paste can only be obtained if the quantity of water circulating within the capillaries is as near as possible to the quantity which evaporates at the surface. The transfer rate of humidity towards the surface of the object being dried depends on the properties of the paste, the dimensions of the capillaries and the viscosity of the water which decreases rapidly when the temperature increases. It is necessary to note also that the surface tension force which causes the displacement of the water in the interior of the capillaries likewise decreases when the temperature increases, but this effect is less important than that'of reduction of viscosity.

If the paste is heated by any process whatever, the movement of the water in the interior capillaries towards the surface is facilitated. This results in regulation of the moisture gradient and shrinkage in the paste. There is thus a need for a process which allows a rapid increase of the temperature of the paste throughout its thickness and once the necessary temperature is reached, to maintain this during the entire surface evaporation. Such a process, which conveniently and selectively heats a humid object in depth, is known. This uses the phenomenon of absorption of radio-electric energy, and particularly of microwave energy, by dielectric losses in the interstitial water of the object.

Due to the use of this process within microwave ovens, only the temperature of the humid object is increased to the almost total exclusion of that of the surroundings and support structure to the object. The support is actually only heated in the zone of contact with the object, where the humidity can diffuse into the adjacent capillaries. In principle, in the case in which the object is a ceramic paste supported'by a mold, the paste should be heated to a temperature as high as possible to increase the speed of transfer of the water. In practice, there is a value of temperature beyond which the plaster of the mold support undergoes a physico-chemical transformation causing dehydration with use and deformations. The paste is'altered and the production output may be decreased by reason of important defectsthat are only noted inthe last stages of manufacture of the porcelain objects. The temperature not to be exceeded is of theorder of 65 C.

In every drying station the calories necessary for the vaporization of the water are drawn from a source of external energy. In the prior drying processes this source is simply the mass of ambient air which circulates slowly around the objects to be dried. Known modern processes use a pulsed flux of dry hot air, having a speed of the order of 2m/s, which may be obtained with infrared heating. If the process of microwave heating is used it is also necessary to employ a drying means, such as ventilation by pulsed air which provides the major part of the vaporization energy. It must be considered that microwave energy is expensive and that its use should be limited as far as possible to the function that it fulfills better than any other, which is to assure the migration of water towards the surface by heating the mass.

SUMMARY OF THE INVENTION The object of the invention is to provide a device for the drying of ceramic pastes, including a source of microwave energy which assures theheating of the paste in its bulk and a source of evaporation energy constituted by a flow of relatively dry air which circulates at the surface of the pastes to be dried.

Another object of the invention is to provide means which allow the drying device to proportion the duration and quantity of the respective action of the microwave energy and evaporation energy in such a way as to consume, for a given rate of production, the least possible of the more expensive microwave energy.

The devices of the present invention permit reduction of the time for drying of ceramic pastes, improving the quality, decreasing the wastage, freeing-the molds very rapidly so that they deteriorate-much less quickly than when used in usual drying stations. There results an important decrease in the number of molds necessary and a great reduction in thefloor space occupied by the equipments.

According to one feature of the invention, molds loaded with ceramic paste are placed in a metallic enclosure comprising first and second cavities separated by a metallic partition. The heating of the paste is produced in the first cavity under the effect of radiation from a first set of microwave sources made up, for example, of magnetrons. A sufficient and homogeneous temperature in the paste is maintained in the second or evaporation cavity, under the effect of radiation from a second set of microwave sources also made up for example'ofmagnetrons. Each cavityis supplied with its own ventilation device of pulsed dry air, the flow of which is regulatable, and the two flows can be very different.

According to another feature of the invention, the two cavities can be raised and lowered together, which allows the loading of a batch of molds furnished with ceramic paste in the first cavity. In normal production conditions there will always be one batch of n molds in the first cavity and another batchof n molds in the second cavity. Each time that a batch of molds furnished with wet paste enters the raised first cavity, a batch of molds containing ceramic pieces suitably dried comes out of the second cavity. The displacement of the batches is assured by a system of discontinuous stepped advancement, the movement of which is controlled and initiated from a program chamber in which the duration of the lowering of the set of cavities, the microwave energy to be dissipated in each of the cavities and the two flows of evaporation air, are set for each step.

According to another feature of the invention, the molds are supported by a metal band with discontinuous advance. The two cavities are open at the lower part and when lowered, the walls, which are provided with a metallic braid, come to rest, under pressure, on the metal band. This device considerably lessens parasitic microwave radiatiomthe pressure being regulated in such a way as to obtain the minimum parasitic radiation. The magnetrons of the microwave sources are automatically put into operation when the pressure is sufficient due to a control in the program chamber.

According to another feature of the invention, the magnetrons and their coupling circuits with the two cavities are supported by the ceiling of the latter. n the ceiling of each of the cavities, there are also fixed pulsed air inlets in the form of adaptation cones which cover all the available surface to the exclusion of that which is occupied by the magnetrons. The cooling of the magnetrons is provided by independent ventilation. The pulsed air penetrates into each of the cavities via a multitude of small holes whose linear dimensions are very much less than the microwave length (cut off holes). The pulsed air, sweeping over the objects to be dried is evacuated by an extraction device made up of adaptation cones placed on the lateral boundaries of the two cavities and aerodynamically coupled to the interior of the latter by a multitude of cut-off holes. The flow of incoming and extraction air can be regulated in each of the cavities by shutters whose movement is controlled in the program chamber.

According to another feature of the invention, the set of sources which feed the first heating cavity with microwave energy, preferably supply varying operating periods and a power substantially higher than the set of sources which feed the second evaporation cavity, whilst the flow of ventilated air in the heating 'cavity is substantially lower than the flow of ventilated air in the evaporation cavity.

According to another feature of the invention. the two cavities have independent microwave sources and relatively incoherent phases. This independence results mainly from the fact that the objects to be dried con stitute a high-absorbent and low reflecting layer at the bottom of the cavity as regards microwave energy. The use of several well distributed independent sources reduces the risk of standing waves, which is much less as the number of sources increases.

In one preferred embodiment of the invention, the magnetrons feeding the first cavity are four in number distributed on the diagonals of the rectangular ceiling and the magnetrons feeding the second cavity are three in number distributed on a single diagonal of the ceiling. These dispositions of the magnetrons are chosen in such a way as to assure that the density of microwave energy absorbed by the humid pastes is almost equal at all the points of the space occupied by the loaded molds.

The objects and advantages of the present invention will appear more clearly from the following description of several embodiments made in relation to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a schematic view of a set of drying equipment according to the invention,

FIG. 2 shows an enclosure with two parallelepiped cavities of equal dimensions having magnetrons supported on the ceilings and coupled to the cavities,

FIG. 3 shows the detail of a circuit coupling a magnetron with a cavity,

FIG. 4 shows devices for entry and extraction of pulsed air according to the invention,

FIG. 5 shows a curve of temperature of the ceramic paste during passage in a drying device according to the invention,

FIG. 6 shows a detail of the embodiment of one variant of the device of FIG. 4,

FIGS. 7, 8 and 9 show support devices for the molds, and

FIG. 10 shows a detail of the system for extracting dried pieces.

. DESCRIPTION OF THE PREFERRED EMBODIMENTS ln order to better'understand the invention it is necessary to review some elements of the known theory of drying. Microwave heating has the effect of constantly moistening the surface of the object to be dried so that the problem becomes that of removal of the surface water by evaporation. There have been many attempts to solve this problem, of which-the technique of drying linen in free air is a well known example. In this latter case it is known that drying is much more rapid with dry air and also with increased wind speed. The laws of surface evaporation by air convection have been well studied. Simple semi-empirical formulas allow calculating the quantity U of water evaporated and the calories furnished by the'air for this evaporation, as a function of: t

the surface S ventilated, a diffusion coefficient k increasing with the speed of the air which touches the surface, the saturation vapor pressure p at the temperature 0,

of the surface water, the partial pressure of watervapor p in the air at temperature 0 far from the surface, the pressure of the air 12,, I

the latent heat of vaporization of water r, a convection coefficient 1 which also increases with the speed of the air. These formulas are written:

(formula A) U sk fpz po Q rU rSk pr-P/Po (formula 5 Q z 0 (formula C) las B and C are considered, it is noted from reference to known theories, that with 0 increasing proportionately to U, it is necessary to use hotter and hotter air. In the known drying problem the previous heating in ventilated air can cause problems that it is possible to avoid by providing air, which is sufficiently dry, at the temperature of the workshops and by more rapid ventilation. In the particular case studied here, in which the mass of the paste is heated, the movement of water towards the surface is much greater with a lower saturation vapor tension p, at the surface. In this case then it is desirable to use air at a temperature 0 which is not too high. If the surface water is at a temperature 0, lower than that within the paste,'the assembly of the latter will tend to cool step by step by conduction. This is why it is necessary, during evaporation, to continue to heat the paste in such a way as to keep the temperature substantially constant.

As shown in FIG. 1, the drying equipment using generally. It supports a lower section la including a transfer assembly having a motor 2 which controls a driving device 3 and a transporter band 4 of stainless steel which rolls around pulleys 5 and 5. The upper part lb, carries lifting elements like a hydraulic jack 6a, integral with an assembly of beams 6b, allowing the pressure forces to be uniformly distributed, and controlled by a hydraulic power group 7. lb also carries the power supplies 8 for the magnetrons, a source of pulsed air ventilation 9, an extraction output 9' for air, and a program chamber 10 in which are stored all the control instructions for the different units of the equipment.

At the interior of lb and supported by 6b is found a microwave enclosure 11 in parallelepiped form. It is generally in duraluminum or of another good conducting material and is of a height between three and six times the height of the product to be dried in molds. The other dimensions depend on the quantityof the product to be treated in each batch of plates to be dried. The enclosure is divided into two equal cavities l4 and 15 by a metal partition 13. On the roof of the cavities are disposed microwave sources 12 or 12 each constituted by a magnetron associated with a microwave circuit coupled by slots to the corresponding cavity. Enclosure 11 and the units that it carries can be raised by the hydraulic jack 6a and the assembly of beams 6b.

Each power supply 8 is connected to the correspond ing magnetron 12 or 12 by'flexible cables 16. The piping for directing ventilated air on the objects to be dried is also supported by'the roof of the enclosure 11. It is not shown in FIG. 1 but will be described below in connection with FIG. 4. The pipes are connected together by bellows at the pulsed air source 9.

The assembly lb is surrounded by a protective grid not show. The doors l7 and 17' allow the introduction into the furnace of objects to be dried or the removal from the furnace of dried objects when the enclosure 11 is raised. During periods of microwave radiation, the enclosure 111 is lowered and comes into contact with the metal band 4 through a metal braid l8 soldered on the edges of the open face of the cavities l4 and 15. The force of pressure of the enclosure ll on the band 4 is controlled by the hydraulic jack 6a and measured by means of a dynamometer now shown.

FIG. 2 shows the parallelepiped enclosure 11 divided by the partition 13 into two cavities l4 and 15 of equal dimensions. On the roof of the cavity 114 are placed four microwave sources 12 each composed of a magnetron coupled to a microwave circuit constituted by a section of wave guide and itself coupled by windows to the cavity 14. For reasons of simplicity the representationv of each source l2 has been limited to a parallelepiped box surmounted by a cylinder which symbolize respectively the microwave circuit and the magnetron.

FIG. 3 provides an exploded view of a known example of a source 12. The magnetron 12a provided with its magnet 12b is terminated by an antenna which couples the magnetron to the microwave circuit 124 made up of a section of wave guide short circuited at its two ends. 12d is coupled to the cavity M by four windows ll2e. Pipes 12f provide circulation of cooling water for the magnetron 12a. Associated with each source 12 is a well known stirrer device, not shown in FIG. 3. It includes a moveable reflector, made up of turning plates which move across windows 12a. The stirrer" moves the standing waves that are produced in the cavity loaded with the material to be heated so that the waves do not keep the same configuration in time. This reduces the effects of an unequal distribution of the microwave energy.

Referring now to FIG. 2, the four sources 12 are placed in diagonal pairs on the roof of the cavity 14. The center of each of the boxes which symbolize the microwave circuits of the sources is situated at approximately mid-distance from the center and from one of the vertices of the rectangular roof. On the roof of the cavity 15 are placed three microwave sources 12 which may be the same as sources 12. The three sources 12 are placed on one of the diagonals of the roof of the cavity 15. The center of one of the boxes which symbolize the microwave circuits 12' is the same as the center of the rectangular roof. The center of each of the two others is situated approximately at two thirds of the distance between the center and one of the vertices of the rectangular roof. The disposition of the microwave sources radiating in the two cavities 14 and l-and variants of these positions will be described later.

FIG. 4 shows the enclosure 11 with a ventilation system. On the upper part of the cavity are found four air inlets 19 of relatively large dimensions, only three of which are shown. The four inlets 19 and four adaptation cones 20, which transmit'air between the inlets and cavity 14, cover the entire surface of the roof 14 surrounding the microwave sources 12. In other devices, the four cones 20 cover only the surface of the roof left free by the sources 12. The transmission of air between the cones 20 and the cavity 14 is made through the roof by means of small holes 21, called cut-off holes. The linear dimensions of the holesare made small compared with the length of a microwave to avoid any parasitic radiation outside the cavity. The dimensions of the holes 21 and their distribution are in such that the flow of air arriving in the cavity shall be as uniform as possible. The four inlets 19 are branched from a larger pipe 22 which is connected through bellows 23 to the source 9 of pulsed air of FIG. 1, this source being provided, for example, with a suction ventilator.

Four shutters 24, each associated with one of the air inlets 19, allow adjustment of the quantity of air introduced in each of the cones 20. Air penetrating into cavity 14 is extracted through further adaptation cones such as 25 which are distributed around the base of the cavity 14 on the three accessible vertical walls. The cones 25 are coupled aerodynamically with the cavity by a multitude of small cut-off holes 26. Pipes 27 connect the cones 25 to a larger pipe, not shown, and through a bellows to the air extraction device 9' of FIG. 1, which device is provided with an extraction ventilator. Shutters 28 allow adjustment of the quantities of air extracted by each of the cones 25.

Cavity l5 incorporates the same apparatus including air inlets 19, adaptation cones 20, cut-off" holes 21, shutters 24, with the air inlets 19' forming branches of the large pipe 22. In the same manner adaptation cones 25' are coupled aerodynamically to the cavity by holes 26', pipes 27 connect the cones 26' to the larger pipe which is connected to the air extraction device 9' of FIG. 1. Shutters 28' allow adjustment of the quantities of air extracted by each of the cones 25. The respective openings of the shutters 24, 24, 28 and 28' allow different flows of air in the two cavities 14 and 15 to be obtained. The magnetrons of the microwave sources 12 and 12 should preferably be cooled by a fluid circulation independent from that which causes evaporation of the water. Thus on the source 12' at the right of FIG. 4 there are shown two pipes 29 which carry water for cooling the magnetron of 12.

In the case of magnetrons cooled by air, it is possible to join the ventilation for the evaporation with the ventilation for the cooling of the magnetrons. FIG. 6 shows schematically an example of double ventilation. This includes an air input 19, an adaptation cone 20, cutoff holes 21 and a source 12. The partial flow of air, derived from the main flow, which goes to cool the magnetron 12 is guided by an adaptation cone 30 covering the assembly of the source 12, A shutter for regulating the partial flow of air may be placed at the apex of 30.

The following description is of the support units for the ceramic pieces to be dried. As shown in FIG. 1, the pieces enter by a door 17 and leave by door 17. At the start of the operation the pieces coming from the machine which shapes the paste and places the plaster molds are collected in batches on the metal band 4 at the left of the drier. According to the first embodiment, shown in FIG. 7, the molds 31' carrying plates 32, for example, are arranged on a platform 33 of low dielectric loss plastic material, polypropylene, amongst others, having extensions 34 of the same material. The platform 33 supported by the extensions is placed on the metal band. FIG. 8 shows another embodiment in which the metal band is providedwith elements 35 which support the platform 33, 33 and 35 also being of plastic material. FIG. 9 shows a variant of the support of molds loaded with paste. The band 4 is replaced by a fixed metal plate 36 on which can be lowered the enclosure 11 to contact the plate through the metal braid 18. Two metal ribbons 37 and 37 of very small thickness, are provided with elements 38 and 38' and can slide on 36. The plastic material platforms 33 of FIGS. 7 and 8 are supported by elements 38 and 38. The small thickness of the ribbons 37 and 37' allows good contact between the plate 36 and the assembly 11 when the latter is lowered.

The following is a description of the drying process using the equipment and devices illustrated in FIGS. 1, 2, 4, 7, 8 and 9. It is assumed that 600 table plates per hour are to be dried. The pieces coming from the machine for shaping the ceramic paste may have, for example, a mass of 800 g including 600 g of kaolin and clay and 200 g of water. The predrying is finished when the ceramic piece is sufficiently rigid to be extracted from the support mold, which results in the evacuation of g of water per piece. The plaster mold has a mass of about 2 kg.

If it is estimated that the regular evacuation of watervapor requires a temperature for the interstitial water of 25 C to 65 C, whatever the predrying process used, it would be necessary to furnish for each plate, at least sufficient energy to raise the temperature of 200 g of water by 40 C and for vaporization of 80 g of water, or 220 kilojoules. The predrying of 600 plates will thus need at the minimum 132,000 kilojoules or, expressed in kilowattsv per hour, 37 kW/h. In presently known processes the plaster of the molds is also heated, as well as the solid components, kaolin and clay, of the ceramic paste. The plaster, like the solid components, has a specific heat which is abouttwo tenths of that of water. If it is supposed that the solids are also heated by 40 C, the supplementary energy to be supplied per plate is raised to 90 kilojoules, or for 600 plates, 54,000 k! or 15 kW/h. In the known processes the necessary energy will thenbe increased to about 50 kW/h.

Already there is seen an advantage of driers using microwave heating, since the molds do not absorb water, the dielectric losses in the dry plaster are small and the molds hardly heat. The saving in energy for production of 600 dried plates an hour isthen about 38,000 k] or kW/h. Where microwave heating is used, the energy necessary to predry 600 plates an hour is raised to 42 kW/h, including 9 kW/h to heat the paste and 33 kW/hfor the evaporation of 80 g of water per plate, or about 50 kg for the 600 plates. The ideal process would use the microwave source for the '9 kW/h necessary for the heating of the paste and the pulsed air for the 33 kW/h of the vaporization energy.

The method for using the device of the present invention, permits approaching this ideal distribution of energies. As previous, a practical case of predrying 600 plates an hour, will be described. This choice evidently does not limit the scope of the invention. Reference will be made mainly to FIG. 1 and in some cases to FIGS. 2, 4, 7 or 8.

The operation commences when the metal band 4 is stopped. The molds carrying the plates to be dried coming from the special shaping machine, such as a Roller machine, are manually or automatically placed on a platform 33 of plastic material (FIGS. 7 and 8). The platform is placed on the loading area situated at the left of the metal band 4. At the end of a predetermined time, sufficient to permit loading of the platform 33, theenclosure 11 is raised, the band .4 is set into motion and stopped when the platform 33 is in place under the cavity 14. The band4 being stopped,

the enclosure 11 comes down again and comes into contact with 4 through the metal braid 18..A security device allows the operations to continue only if the pressure of the enclosure on the band 4 is sufficient.

When the pressure is suitable, the four magnetrons of the sources 12 on the roof of the cavity 14 are started simultaneously. The three magnetrons carried by the cavity 15 may also be started at the same instant. After a predetermined time t, the operation of the magnetrons is stopped under control of a security or locking device, with the enclosure 11 being raised. When 11 has reachedits maximum height the band 4 is started again and a new batch of plates to be dried, carried by another platform 33, is engaged under the cavity 14 while the first batch passes under the cavity 15.

The operation continues step by step. In general,

when the batch of rank n enters under cavity 14, the

batch of rank (n-l) comes under the cavity 15 while the batch of rank (re-2) passes to the discharge area situated on the right part of the band 4. The sufficiently rigid plates 32 are then separated from their respective molds 31 by pressure from suction pipes 39 which are positioned over the plates, as shown in FIG. 10. The

.10. platform 33 carrying the empty molds is then conveyed towards the special Roller shaping machine by a transporter band.

During all the time of the operations, the pulsed air ventilation and air extraction devices act continuously even when the enclosure 11 is lifted and the magnetrons stopped. The flow of air in inlets 19 or 19, FIG. 4, can be different. The plates situated under cavity 15, in which evaporation is primarily effected, are generally ventilated more than those which are placed under the cavity 14 in which the pieces are mainly to be rapidly heated in bulk. The extraction air flow is a little less than that introduced. When the enclosure 11 is lowered, there is a slight overpressure which assures homogenization of the flow of air on the pieces to be dried. In the example chosen to explain the operation of the device, each batch comprises 30 plate-mold assemblies. The time t of passage of one batch under each of the cavities 14 and 15 is 150 seconds and the time t of the operations of raising the enclosure 11, displacement of the band 4, lowering of 11 and again setting the magnetrons into operation, is 60 seconds. Roughly it can be said that every 3 minutes 30 plates ready to be removed from their molds come out on the right of band 4, which is at a rate of 600 dried plates per hour.

The dimensions of each of the cavities are about 3m in length, lm in width-and 0.6m in height. The flow of pulsed air for the assembly of the two cavities is of the order of 2000m /hour. The four magnetrons of the sources feeding microwave energy to cavity 14, FIGS. 2 and 4, each have a nominal power of 2.5 kW, which, at

an efficiency of percent, corresponds to' 10 kW of power radiated in 14. The three magnetronsof sources 12 feeding cavity 15, FIGS. 2 and 4, with microwave energy also each have a nominal power of 2.5 kW.

Taking. into account the role played by the microwave heating in cavity 15, which is intended to keep constantthe bulk temperature of the plates, the energy required of sources 12' is generally less than that furnished by the sources 12 which radiate into the cavity 14. In order to economize on the expenditure of energy, sources 12' can be made to operate less time during each elementary operation than sources 12. The power .to the magnetron at the center of the roof of cavity 15 can then be less thanthat of each of the two others. This latter solution has another advantage which will be examined later. It also permits obtaining a better distribution of the microwave energy dissipated in the plates placed in cavity 15.

If the seven magnetrons supply the same power during operating periods representing five sixths of the total time, it is noted that the microwave energy consumed rises to 2 X 7 X 5/6 12 kW/h. If this result be compared with the ideal values estimated above, 9 kW/h for the heating of the paste and 33 kW/h for the vaporization of 50 kg of water, it is seen that the microwave energy is used efiiciently in accordance with the system of the invention. The difference of 3 kW/h can be explained by losses in the metal boundaries of the cavities and in the plaster of the molds and also by a small vaporization energy supplied to the internal water, this vaporization being used to facilitate the migration of water toward the surface.

In order to better understand the operation of the proposed system, it is helpful to examine the evolution 1 l of the internal temperature of the paste of the plates during the 6 minutes that the drying of a batch lasts; This evolution is shown by the curve of FIG. which in-. dicates the time T in minutes on the X-axis and the temperature 0 in degrees centigrade on the Y axis. During the period of exposure to the microwave radiation under the cavity 14 the temperature increases regularly from 25 C to 65 C. The slope of the curve is determined by a suitable distribution between the microwave energy and the evaporation energy carried by the ventilated air. During the minute of lifting of the enclosure 11, the plates are subjected solely to the cooling of the ventilated air and the temperature of the paste decreases slightly. Lastly, during the period of exposure to the microwave radiation under the cavity 15, the temperature again increases and is then kept at a constant value up to the end of the operation. Here again the slope of the curve results from a suitable distribution of the two energies.

It should be understood that the characteristics of the present system for drying ceramic plates of a different nature from the present examples, will be determined as a first approximation by simple empirical laws. With a fixed quantity of water to be evaporated during a unit of time, it is possible to calculate the flow of pulsed air when the surface of the elements to be dried and the temperature of the air are known. This calculation is made by using the fonnula A noted above. The quantity of paste to be heated during a unit of time to reach a suitable temperature, and timing of the operations of raising and lowering of the enclosure, permit determining an approximate value, of the necessary microwave power. In practice, the total power for the assembly of microwave sources, is made greater by 50 percent than that calculated. This microwave power is then distributed between the two cavities by attributing a higherpower to the first cavity 14,.FIGS. l, 2, 4, in which the rapid rise of the temperature of the paste is mainly effected.

One of the important qualities that can be expected of microwave heating is a suitable distribution of power in the elements to be heated. A first known means, is the introduction into the radiation enclosures of move: able reflectors or stirrers which act as wave mixers and limit the effect of standing waves. A second means, which should also accompany the first, consists in using n independent sources in place of a single one. At any point whatever inside a cavity, the n microwave fields are combined but with incoherent phases which limits the risks of appearance of standing waves,n should be made as high as possible.

When the linear dimensions of the cavity are large compared with the wave length, and when the assembly of pieces to be heated covering the bottom of the cavity are presented as an absorbent and low reflecting boundary, everything takes places as if the microwave sources on the roof of the cavity radiate like independent light sources. The radiation diagram of each source, which depends on the nature of the coupling windows between the microwave circuit associated with a magnetron and the cavity, (see FIG. 3), is directive and the elements placed vertically in relation to a source receive more energy than the others. In the case in which four sources are used, as shown in FIG. 2, the most suitable distribution is that which has been described. In the case in which three sources are used, the one placed in the center of the roof of the cavity, as shown in FIG. 2, will preferably have a power substantially less than that of each of the two others.

Many variations are easily effected. If for example eight sources are used, they are equally distributed on the two diagonals with two per half diagonal, the first being situated approximately at one third of the distance between the center of the roof and the corresponding apex and the second approximately at two thirds of the same distance. If five sources are used, the disposition of the three sources shown in FIG. 2 is completed by two other sources placed on the other diagonal of the roof, the central source having, preferably, a power substantially less than that of eachof the other four.

In order to clearly demonstrate the advantages of the system of the present invention, the number of molds and the space occupied by the equipment necessary for predrying 600 platesper hour are listed below.

In the present system:

Number of molds Floor surface 12 m In a known prior system, of the rocking drying type:

Number of molds 2400 Floor surface 120 m Although the principles of the present invention may have been described above in relation to a particular embodiment, it will be clearly understood that the description is given only by way of example and does not limit the scope of the invention.

What is claimed is:

l. A process for drying ceramic articles and molds using microwave heating and air ventilation comprising:

feeding a mold containing a ceramic article in a discontinuous stepped sequence into a first heating chamber having a first microwave radiation source and first source of ventilating air,

subjecting said mold and article to microwave radiation of a first predetermined power level and a relatively light flow of dry air to rapidly heat said mold and article to a predetermined temperature and dry said article,

feeding said mold and article in said stepped sequence into a second adjoining heating chamber having a second microwave radiation source of lower power and second source of a relatively heavy flow of dry ventilating air, said second 'chamber being capable of operating simultaneously with said first chamber, each chamber operating at different radiation and ventilation levels,

applying said second lower radiation and relatively heavy flow of air to said mold and article, said second radiation maintaining said temperature and said heavy air flow evaporating water from said article and stopping said microwave radiation and raising and lowering said chambers to permit movement of said article and mold into and out of said chamber.

2. The process of claim 1, including feeding a plurality of molds containing articles into said first and second chambers in said sequence, different successive groups of articles and molds being fed into respective said chambers simultaneously,

simultaneously operating said first and second cham-' bers at said different radiation and ventilation levels, removing the dry articles and molds from said second chamber. 3. Apparatus for drying ceramic articles and molds comprising:

first and second microwave heating chambers positioned adjacent one another and having a common partition therebetween, first and second microwave radiation means coupling different levels of microwave energy into respective said chambers, first and second air ventilating means directing different levels of air flow into said respective chambers, means feeding a mold containing a ceramic article into said first and second chambers in a discontinuous stepped sequence, said chambers being capable of operating simultaneously on different successive articles and molds, said feeding means includes a discontinuously advancing platform supporting and enclosing the base of said chambers, means providing electrical contact between said platform and chambers, means raising and lowering said chambers relative to said platform to make and break said contact and means for actuating and controlling the application of microwave energy and ventilating air to heat and dry each said mold and article.

4. The apparatus of claim 3, wherein said air ventilating means includes a common source of air and means directing a relatively light flow of air from said source into said first chamber and a relatively heavy flow of air into said second chamber, said microwave means coupling a relatively higher energy level into said first chamber and lower level into said second chamber.

5. The apparatus of claim 4, wherein said microwave means includes a plurality of magnetrons mounted on the roof of each said chamber.

6. The apparatus of claim 5, wherein said air directing means includes air ducts, shutters in said ducts, and adaptation cones positioned over said magnetrons and chambers, a plurality of holes in the roofs thereof, and air extraction means positioned in each said chamber.

7. The apparatus of claim 5, including control means for stopping operation of said microwave means when said contact is broken, said feeding means moving said article and mold from said first chamber into said second chamber during said stopping operation.

8. The apparatus of claim 5, wherein said plurality of magnetrons is greater in number over said first chamber than said second chamber and provide relatively even distribution of said radiation in each said chamber.

9. The apparatus of claim 8, wherein said platform includes dielectric supports and a plurality of articles and molds disposed on said supports, said platform providing movement of groups'of said plurality of articles and molds into and out of said chambers in sequence. 

1. A process for drying ceramic articles and molds using microwave heating and air ventilation comprising: feeding a mold containing a ceramic article in a discontinuous stepped sequence into a first heating chamber having a first microwave radiation source and first source of ventilating air, subjecting said mold and article to microwave radiation of a first predetermined power level and a relatively light flow of dry air to rapidly heat said mold and article to a predetermined temperature and dry said article, feeding said mold and article in said stepped sequence into a second adjoining heating chamber having a second microwave radiation source of lower power and second source of a relatively heavy flow of dry ventilating air, said second chamber being capable of operating simultaneously with said first chamber, each chamber operating at different radiation and ventilation levels, applying said second lower radiation and relatively heavy flow of air to said mold and article, said second radiation maintaining said temperature and said heavy air flow evaporating water from said article and stopping said microwave radiation and raising and lowering said chambers to permit movement of said article and mold into and out of said chamber.
 2. The process of claim 1, including feeding a plurality of molds containing articles into said first and second chambers in said sequence, different successive groups of articles and molds being fed into respective said chambers simultaneously, simultaneously operating said first and second chambers at said different radiation and ventilation levels, removing the dry articles and molds from said second chamber.
 3. Apparatus for drying ceramic articles and molds comprising: first and second microwave heating chambers positioned adjacent one another and having a common partition therebetween, first and second microwave radiation means coupling different levels of microwave energy into respective said chambers, first and second air ventilating means directing different levels of air flow into said respective chambers, means feeding a mold containing a ceramic article into said first and second chambers in a discontinuous stepped sequence, said chambers being capable of operating simultaneously on different successive articles and molds, said feeding means includes a discontinuously advancing platform supporting and enclosing the base of said chambers, means providing electrical contact between said platform and chambers, means raising and lowering said chambers relative to said platform to make and break said contact and means for actuating and controlling the application of microwave energy and ventilating air to heat and dry each said mold and article.
 4. The apparatus of claim 3, wherein said air ventilating means includes a common source of air and means directing a reLatively light flow of air from said source into said first chamber and a relatively heavy flow of air into said second chamber, said microwave means coupling a relatively higher energy level into said first chamber and lower level into said second chamber.
 5. The apparatus of claim 4, wherein said microwave means includes a plurality of magnetrons mounted on the roof of each said chamber.
 6. The apparatus of claim 5, wherein said air directing means includes air ducts, shutters in said ducts, and adaptation cones positioned over said magnetrons and chambers, a plurality of holes in the roofs thereof, and air extraction means positioned in each said chamber.
 7. The apparatus of claim 5, including control means for stopping operation of said microwave means when said contact is broken, said feeding means moving said article and mold from said first chamber into said second chamber during said stopping operation.
 8. The apparatus of claim 5, wherein said plurality of magnetrons is greater in number over said first chamber than said second chamber and provide relatively even distribution of said radiation in each said chamber.
 9. The apparatus of claim 8, wherein said platform includes dielectric supports and a plurality of articles and molds disposed on said supports, said platform providing movement of groups of said plurality of articles and molds into and out of said chambers in sequence. 